(1) Field of the Invention
The present invention relates to an optical amplifying device which is capable of controlling gain of signal light by, for example, introducing a control light, and which is preferably utilized for a semiconductor optical amplifying device.
(2) Description of Related Art
As demand for communications increases dramatically in recent years, scope of application of large capacity and high speed photonic network is spreading to metropolitan access system which is close to subscribers. The photonic network applicable to the metropolitan access system has flexible network structure in which optical add drop multiplexer (OADM) and the like are utilized.
For the optical amplifier utilized in the network of this kind, function by which high speed gain control (level control function) can be performed against variation in number of multiplied wavelengths and variation in intensity of input light to get constant light output at any time is required.
As for the optical amplifier device with level control function, for example, as shown in FIG. 10, an amplifier which is configured in such a manner that semiconductor optical amplifiers (SOA) 103, 104 are disposed respectively in both arms 101, 102 of a Mach-Zehnder interferometer 100 which includes 3 dB couplers 107, 108, and a control SOA 105 is integrated outside of the interferometer 100, has been proposed (For example, see Japanese Patent Laid-Open No. 2003-179289).
In this optical amplifier, signal light is amplified by the SOAs 103, 104 which are disposed in both arms 101, 102 of the Mach-Zehnder interferometer 100. Further, laser oscillation occur in the device by means that: a light path for oscillation light (control light) is arranged in the device such that it intersects with a light path for the signal light, and reflection preventing film (reflection mirror) 106 which has reflection coefficient about 0.1% and a gain medium such as SOA are disposed on light path for the oscillation light, as feedback mechanism. Here, as for other feedback mechanism, a diffraction grating, a loop waveguide and the like are also disclosed. In this case, SOAs 103, 104 in the arms 101, 102 also contribute to the laser oscillation of oscillation light, and gain for signal light is arranged to constantly be held (clamped) in gain for oscillation light at oscillation threshold (threshold gain). Further, in this optical amplifier, threshold gain of the SOAs 103, 104 in the both arms 101, 102 are changed by means of adjustment of gain of the control SOA 105 which is disposed outside of the interferometer, thereby gain control of the optical amplifier is accomplished.
Especially in the optical amplifier disclosed by the Japanese Patent Laid-Open No. 2003-179289, at a time point when gain of the oscillation light in one way becomes a reciprocal number of residual reflection coefficient at end face, the oscillation begins and the gain is clamped. If residual reflection coefficient of the reflection preventing film is 0.1%, at a time point when sum of gains of the SOAs 103, 104 in the arms 101, 102 and gain of the control SOA 105 becomes 30 dB, the gain is clamped. Therefore, it is described that as gains of the SOAs 103, 104 in the arms 101, 102 are clamped at 20 dB if gain of the control SOA 105 is set to 10 dB, gain of the SOAs 103, 104 in the arms 101, 102 are adjusted by means of the gain control of the control SOA 105 (See, for example, paragraph numbers 0046 and 0047).